How do I unwrap an over-complex mesh?
Normally such mesh should be retopologized.

But anyway if for whaterever reason you don't want or cannot do a propper retopology, you can just use Blender's automatic unwrapping features.

When you select your mesh in edit mode and press U, it brings up the UV mapping menu with plenty options.



In red I marked the automatic unwrap methods. They each automatically unwrap your selected mesh following different sets of rules and eventually with some options you can have a hand on. They each have some ideal situations they're good at.

The one you might like here is the Smart UV Project. It basically makes seams when adjacent faces form an angle superior to a chosen value (the default is 66).



You can click that and see how it goes. If there are too many cuts, lower the angle limit, vice versa. Try to find the right balance between the number of islands and the obtained distortion. If it's not an important mesh (which I guess it is if you don't retopologize it), you don't have to spend time on that, just make something that looks ok from afar and move on.

Learn more in the manual.